1. Field of the Invention
The present invention relates generally to a semiconductor structure and a process thereof, and more specifically to a semiconductor structure and a process thereof, that forms a carbon-containing silicon germanium cap layer on an epitaxial layer.
2. Description of the Prior Art
For decades, chip manufacturers have made metal-oxide-semiconductor (MOS) transistors faster by making them smaller. As the semiconductor processes advance to very deep sub micron era, such as 65-nm node or beyond, how to increase the driving current for MOS transistors has become a critical issue.
In order to improve device performances, crystal strain technology has been developed. Crystal strain technology is becoming more and more attractive as a mean to obtain better performances in the field of CMOS transistor fabrication. Putting a strain on a semiconductor crystal alters the speed at which charges move through that crystal. Strain makes CMOS transistors work better by enabling electrical charges, such as electrons, to pass more easily through the silicon lattice of the gate channel.
In the prior arts, attempts have been made to use a strained silicon layer, which was grown epitaxially on a silicon substrate with a silicon germanium (SiGe) layer disposed in between. In this type of MOS transistor, a biaxial tensile strain occurs in the epitaxial silicon layer due to the silicon germanium having a larger lattice constant than the silicon one, and, as a result, the band structure alters, and the carrier mobility is increased. This enhances the speed performances of the MOS transistors.
However, ingredients of the epitaxial layer are complex and will diffuse easily and pollute the peripheries during subsequent processes.